Coating metal



March 19, l

Filed Oct. 24, 1941 WHITFIELD ET AL COATING METALS '2 Sheets-Sheet lFIG. 6.

FE'G. 5.

March 19, 1946. M, G. WHITFIELD ET AL 2,396,730

coAT'ING METALS Patented Mar. 19, 1946 COATIN G METAL Marshall G.Whitfield and Victor Sheshunoi,

Garden City, N. Y., assignors to Al-Fin Corporation, Farmingdale, N. Y.,a corporation of Delaware Application October 24, 1941, Serial No.416,328

7 Claims.

This invention relates to the coating of metals with other metals, andhas particular reference to the coating of ferrous metals with aluminumor its alloys.

Heretofore Various methods have been proposed for coating ferrous metalswith aluminum with a view to procuring a composite structure which wouldwithstand mechanical deformation, wide variation temperature, and thelike, without separation at the zone between the two metals. Thesemethods involved dipping the ferrous metal in molten aluminum, castingthe aluminum on the ferrous metal, or spraying'the ferrous metal withmolten aluminum with or without the use of fluxing or bonding materials.Few of these methods have been sumciently satisfactory to `justify theircommercial use for all purposes.

Also, these prior methods were complicated and expensive and the resultsof their use were so variable that production of uniform products wasnotat all certain.

In accordance with the present invention, a process is provided forapplying a coating of aluminum, or aluminum base alloys, and othernon-ferrous metals having similar characteristics, to a base metal, suchas one of the iron or steel metals, to any desired degree of thicknesswhile at the same time producing a bond between these metals .which willwithstand deformation, wide temperature changes and other rigoroustreatment.

Also in accordance with the present invention, particularly when appliedto the coating of steel, the process may be utilized to heat-treat thesteel to temper and harden it without the use of any other steps thanthose normally involved in the coating operation.

The preferred process of the invention is conducted essentially in foursteps, beginning with a thorough cleaning of the base metal of allforeign matter, as by brushing or sand-blasting, to remove scale, dirt,and other material which would interfere with the formation of a strongbond. A chemically clean surface is not required; it is necessary merelyto clean the surface suiiiciently to remove scale, dirt and greasetherefrom.

If only part of the ferrous article is to be coated,

the portions not to be coated are covered with a suitable stop-offpreparation which will prevent contact between those portions and thealuminum, or aluminum base alloy, and the article is then immersed in abath of molten aluminum, or aluminum base alloys, for a suicient timeand .at a proper temperature to cause the aluminum to alloy with theferrous metal and form a thin ironaluminum alloy bond on the surface ofthe article. Preferably this operation is of short duration in order topreclude deep alloying action on the surface of the article, inasmuch asthe iron-aluminum alloy formed thereupon is a hard, brittle metal whichis not capable of withstanding `considerable shock or distortion, andhence should not be so thick as to weaken the eventual bond.

A third step of the process consists of immediately transferring thearticle coated with the thin film of molten aluminum, to a second bathfor applying a second lm to the article. This second bath preferably hasa lower melting point than aluminum or aluminum alloy of the first bath,and is maintained at a temperature below the criitcal temperature of thearticle, thereby also serving as a quenching medium for hardening andtoughening the metal, but not so low as to cause the initial aluminumcoating to freeze to any substantial degree. An alloy of aluminum and atemperature-depressing metal or metals has proven effective for thatpurpose. The article is retained in the quenching bath until itstemperature has attained substantially the temperature of the quenchingbath, and then a layer of aluminum of any desirable thickness, isapplied over the wetted surface remaining after removal from thequenching bath. This aluminum layer maybe any desired thickness, and ispreferably applied by casting it on the article, preferably by placing amold about the wetted surface of the article and pouring molten aluminumin contact therewith to the desired depth or thickness. When thealuminum solidies, a substantially homogeneous layer of aluminum isprovided on the surface of the article which is secured by the bond offerro-aluminum alloy to the ferrous article.` This layer of aluminum maythen be machined or otherwise treated to produce the iinished orsemi-finished product desired.

As an example of one use of the method of this invention, 'an internalcombustion engine cylinder, including barrel and head as a unit, orbarrel and head separately, may be provided on its outer surface with amuff of aluminum in the manner described and the muif provided withgrooves or slots forming cooling fins, these grooves or slots eitherbeing formed by casting when the aluminum is applied to the cylinder, orby machining the aluminum muff so formed, as by saw-cutting or millinggrooves in the metal. Cylinder barrels formed of appropriate types ofsteel and treated in accordance with the process outlined above arecharacterized by a high Rockwell hardness,

toughness, and by a firmly seated and chemicallybound cooling structureaiiixed thereto.

It will be seen that the method of the present invention provides a verysimple, inexpensive and enlcient method for applying to a ferrous basemetal article a layer or plate of aluminum, or an alloy thereof, to anydesired thickness which may be used for the purpose of providing thebase metal with added properties or facilities, such as, for example,the rapid cooling afforded by the aluminum because of its higher heatconductvity.

For better understanding of the invention, reference may be had to theaccompanying drawings, inwhich:

Figure 1 is a view in vertical section illustrating the manner in whicha cylinder barrel may be coated by dipping the same in a bath of moltenaluminum;

Figure 2 is a view in vertical section of a typical form of mold forcasting aluminum about the cylinder barrel;

Fig. 3 is a plan view of the mold of Fig. 2;

Fig. 4 is a view in vertical section of a cylinder barrel with thealuminum mui! applied thereto;

Fig. 5 is a view in vertical section of the cylinder barrel with themuil` treated to provideY engine is treated to produce a iinishedconstruction having cooling ns I I thereon, as shown particularly inFig. 5 of the drawings. The particular cylinder barrel I0, shown by Wayof example, may consist of a tubular member of any desired dimensionshaving a radially directed flange i2 thereon and `a suitably raisedannular ring I3. No special surface finish, such as roughening, need beprovided to aid in securing the adhesion or binding of the aluminumthereto.

Various types of steel may be used in the preparation of such cylinderbarrels, typical of which is high strength SAE 4140 steel (achromemolybdenum steel, containing 0.8l.1% chromium, O-0.25% molybdenum,G35-0.45% carbon, 0.60-0. 90% 0.04-0.05% phosphorous and silicon each).

Inasmuch as it is desired to coat only a portion of the exterior of thebarrel I0, namely, that portion above the flange I2, the remainder ofthe barrel, including the inside and the flanges, may be coated with astop-off preparation, such as, for example, a lime solution orequivalent coating. The barrel is then supported on a ring I4 which maybe formed of steel, iron, or other material capable of withstanding thehigh temperature of the coating bath and which is provided with a handlel5 by means of which the cylinder I0 may be lowered into bath I6; Thebath IE which may bean externally heated crucible lia, or the like, ispartially filled with molten aluminum, or an aluminum base alloy, andmaintained at a temperature between about 1525 and 1625 F. 'I'hetemperature `of the bath I6 depends upon the melting point of thealuminumcontaining metal of which it is composed, but in any case itstemperature is maintained somewhat above its melting point, asindicated.

An annular collar or sleeve l1 is supported loosely on the upper surfaceof the ange i2 of the cylinder barrel and is lowered into the moltenmanganese, and not more than ascenso aluminum with the cylinder barrel l0. The collar I1 is provided with outwardly projecting arms il whichengage the top edge of the crucible Ila so that as the cylinder barrelI0 is lowered into the bath the collar Il will be stopped by engagementof arms Il with the top edge of the crucible lia. drops below the collarI1 and allows the molten aluminum to flow between its lower edge and theflange i2 on the cylinder barrel.

The function of the ring I4 and the collar Il is to part and push asideand thus prevent contact of the slag or aluminum oxide on the top of thebath with the portions of the cylinder barrel l0 to be coated. Thus, asthe cylinder barrel I0 and the ring l5 are lowered into the bath II thealuminum oxide will be parted and will flow around the cylinder barreland upwardly around the collar I1. When the collar I1 and the flange i2separate, the molten metal beneath the oxide scum ilows around the upperportion of the cylinder barrel and comes into intimate contact with it.

Temperatures between about 1525 and 1625 F. are used to promote a veryquick formation of a ferro-aluminum alloy at the engaging surfaces ofthe bath I6 and the cylinder barrel l0. A sufficient bond between thealuminum and the cylinder barrel l0 will be obtained within one to sixminutes, or in the case of small articles, such as, for example, wire,within a few seconds. Inasmuch as only a thin layer of theferro-aluminum alloy is desirable in most instances, the formation ofthe alloy is arrested by cooling the coated barrel I0. This may beaccomplished by immersing it in a second bath of molten aluminum or analuminum base alloy, at about the melting point of alumium or slightlybelow, preferably between 1000 and 1250 F. Preferably a bath containingaluminum and silicon is used, with the silicon depressing the meltingpoint of the aluminum. For example, a suitable bath may contain about13% silicon. The use of such an alloy bath also has the added advantageof cleaning the surface of the aluminum coating on the cylinder barrel.It appears that when the highly heated barrel is immersed in the lowertemperature bath, the heat of the barrel raises the temperature of thebath adjacent the barrel surface and the consequent increased fluidityof the bath washes or strips the oxide from the surface of the aluminumcoating. AThe second bath thus arrests the alloying action and alsoplaces the surface of the barrel in condition for the subsequent coatingoperation. The reduced temperature of the barrel also facilitates vthesubsequent coating operation because the coating temperature is morenearly equaled.

During the initial coating operation in bath I6, the cylinder barrel I0is heated to a high temperature, such that solution ofthe constituentsof the steel takes place, thus placing the entire barrel in a conditionto be hardened by subsequent quenching.

Inasmuch as it is known that many steels, when allowed to cool in air orquenched in oil from temperatures on the order of 1525 F, will form anextremely hard but brittle martensitic crystalline structure in themetal, advantage is taken of the use of the second bath to quench thesteel rapidly to a temperature below the critical, i. e.

below the beginning of precipitation of ferrite from the austenite oncooling. By regulating the subsequent treatment ofthe steel, the desiredproperties are obtained. This quenching opera..

The descending barrel i0 thusl tion results from the immersion of thebarrel in the second bath of aluminum or an aluminum base alloymaintained at a temperature below the critical temperature of the steelinvolved, but preferably above martensite formation temperature,

A bath of this type has several exceedingly valuable advantages, namely,(1) a rapid quench which results from the especially good heatconductivity of the molten aluminum or aluminum base alloy, of which thesecond bath is composed, (2) the great .heat extraction capacity of thealuminum and aluminum base alloys enables the close control of thequenching temperature.

It willbe observed that both in the first bath and in the second baththe steel is perfectly wetted by the aluminum or aluminum base alloy ofthe bath, so that there is virtually a continuous heat transfer pathbetween the interior of the steel member and the highly conductive bath,whereby travel of heat in and out is facilitated. This method thuscontrasts with lead and other molten metal bath quenching methods wherethe article to be heat treated is not wetted and hence heat transmissionis imperfect at the interface.

Inasmuch as the steel has been perfectly wetted with aluminum in thefirst coating bath, immersion in the second bath at a lower temperaturearrests the otherwise rapid formation of the ferro-aluminum alloy bond,and in addition hardens the cylinder. Cylinders treated in accordancewith our process have, lin many instances, a hardness of 55-58 RockwellC,

When the article has been quenched in the second bath, and the formationof the ferro-aluminum bond arrested, the cylinder barrel, while stillwetted by molten aluminum, is transferred to a mold where moltenaluminum at its melting point, or slightly above, is poured about thewet cylinder barrel and cooled to form the aluminum muli 30 of anyrequired thickness on the exterior surface of the barrel, as shown inFig. 4. The quenching in the second bath purposely reduces thetemperature of the article below that of the casting metal to precludereheating of the article during casting to a temperature where the steelstructure will be changed. The steel is accord- Y ingly not reheated toits critical temperature. Where heat treatment of the steel is notnecessary or desirable, the second or quenching bath may be omitted, andthe muff 3|] cast immediately on the aluminum skin overlying the thinferroaluminum alloy formed on the article l in the first bath and whilesaid skin is still Wet as described.

Figs. 2 and 3 of the drawings disclose a suitable type of mold M forcasting the aluminum muif 30 on the cylinder barrel I0. This mold may beof the split type, including two segments 20 and 2l, in the form ofhalves of a hollow cylinder having overlapping hinge flanges 22 and 23at the edges thereof, which are joined by hinge pins 60 permit the twosections to be spread apart. Adjacent the lower end of the mold M ineach section 20 and 2| is an inwardly projecting shoulder 24 forming anannular support for the flange I2 on the cylinder barrel. A taperingopening 25 is also provided for receivingthe lower end of the cylinderbarrel I0.

The cylinder barrel l0, still wetted by the molten aluminum, or aluminumbase alloy, is placed in the mold M with an asbestos washer 26interposed between the flange l2 thereon and the shoulder 24 in theinterior of the mold. Molten aluminum can then be poured into the upperend of the mold between its wall and the outer wall of the wet cylinderbarrel I 0 to produce a muli 30 which is bonded securely to the cylinderbarrel I0 by the alloy bond described above.

The casting may be cooled by blowing air through the interior of thecylinder barrel I0, or quenched in oil, and when the aluminum ishardened, the mold maybe split to permit removal of the cylinder barrel.

Thereafter the aluminum muif 30 on the barrel may be machined in anysuitable way to cut slots therein and to remove unwanted portionsthereof to form cooling fins Il which are bonded tightly to the cylinderbarrel, and thus are capable of quick and efficient transmission of heatfor cooling purposes.

If desired, prior to machining of the muff 30 on the cylinder barrel,the article may be transferred to a holding furnace for temperingpurposes and thereafter quenched in oil to temper it.

It will be understood that the temperatures of the various baths and thetimes of treatment of the articles being coated and hardened will varyconsiderably, depending upon the size of the article, and hence its rateof heat transfer, and the type of metal undergoing treatment. However,it has been found that the most satisfactory temperatures for most typesof steel in the initial alloying and coating bath will lie in therange'between 1525 and 1625 F. The time of immersion in this bath varieswidely. For example, in coating wire it has been found that a fewseconds are oftentimes sufficient to successfully form a strongferro-aluminum bond, whereas larger articles may require as much as tenminutes or even more to heat the surface to be coated to the alloyingtemperature necessary to form a satisfactory bond.

The quenching temperature also may be varied, depending upon the type ofcrystalline structure to be formed in the metal, but we have found thata temperature between 1000 and 1250 -F. is, in most instances,satisfactory for the treatment of steels. The duration of the quenchingstep will, of course, vary considerably, depending upon the size of thearticle and the rate at which the heat can be removed therefrom.Ordinarily, with a cylinder barrel of the type described above, aquenching period of one-quarter to five minutes has been found to beentirely satisfactory.

As illustrated diagrammatically in Fig. 6, the sequence of steps of theprocess may be varied slightly when our process is used only in the heattreatment of steel. As shown in this diagram, the article, for example,a piece of steel, is raised in temperature to between, about 1575 and1625 F., then quenched at about 1100 F. in an aluminum-silicon alloybath. The quenched article is then quenched in oil, drawn at about 1000F., and quenched again in oil. Optionally, depending :upon the hardnessand other physical properties required, the article may be placed in adraw furnace, after the rst quench in the second bath, for a suitableperiod, followed by an oil quench. This is indicated by the dotted curveof Fig. 6.

The high heat conductivity of aluminum at the temperatures involved(1.01 at about 1110) and the high specific heat of aluminum (.2683)provides a particularly rapid quenching and heat dissipating action andthus allows accurate con-` trol of the heat treating operation.

From the foregoing disclosure of the present invention, it will be clearthat we have provided articles with aluminum and, if desired or requiredsimultaneously heat treating such articles to produce a high hardnesswith adequate toughness to withstand the rigorous treatment affordedsuch articles as cylinder barrels for aeronautical engines,notwithstanding the high temperatures and widely -diiferent coefficientsof thermal expansion of the aluminum and the steel.

It will be understood from the foregoing that the process disclosedherein is susceptible to considerable variation in the temperatures andtimes of treatment, depending upon the characteristics of the articlesbeing treated, its composition, and the physical characteristics desiredin the finished products. Therefore, the, embodiment of the inventiondisclosed herein should be considered as illustrative only and not aslimiting the scope of the following claims.

We claim: i. Y

1. A method of directly coating a ferrous metal article withaluminum-containing metal of the class consisting of aluminum andaluminum base alloys, which comprises heating the surface of saidarticle to be coated to a temperature 'not materially in excess of 1625F. and above the melting point of said aluminum-containing metal tocause the ferrous metal of said surface to alloy with the saidaluminum-containing metal in contact therewith in the molten state,maintaining said heated ferrous metal surface of the article and saidmolten aluminum-containing metal in contact with each other for a periodof time only sucient to form a film of ferro-aluminum alloy at theinterface between them and an initial coating of saidaluminum-containing metal on said alloyed surface of the article,casting between said initial coating and a mold spaced therefrom a layerof molten metal of the class consisting of aluminum and aluminum basealloys while said initial coating and said surface are at a suiiicientlyhigh temperature for'said surface to be wetted by said initial coating,and freezing said coating and layer on said surface whereby said coatingand layer are united to form a single coating of saidaluminum-containing metal of the required thickness integrally bonded tosaid surface of the article through said ferro-aluminum alloy.

A method of coating metals which comprises immersing a ferrous metalarticle in a bath of molten metal of the class consisting of aluminumand aluminum-base alloys at a temperature and for a time suiiicient toform a ferro-aluminum alloy film on the surface of said article,immersing said article without cooling in a bath of molten metal of theclass consisting of aluminum and aluminum-base alloys maintained at alower temperature than said first bath to arrest alloy formation, toclean the surface o f said nlm and form a molten coating on saidarticle, and thereafter casting a molten metal of the class consistingof aluminum and aluminum-base alloys on said coated surface While thesurface of said article is still wet with said molten metal from saidsecond bath.

3. A method of coating metal with an aluminum-containing metal of theclass consisting of aluminum and aluminum-base alloys, which comprisescontacting a ferrous metal article with molten aluminum containing metalat a tem- 7 perature between about 1525 and about 1625 F. for a periodof time suiiicient to form a lm of' ferro-aluminum alloy on the surfaceof said article, immersing said article without substantial cooling andwhile wet with molten aluminum conascenso taining metal ina bath of analuminum-silicon alloy maintained at a temperature between about 1000 F.and about 1250 F. to cool said article, arrest further alloy formationand clean the surface of said film, and subsequently applying to thesurface of said article while at substantially thelatter temperature acoating of molten aluminum containing metal, and cooling said article.

4. The method of coating a ferrous metal engine cylinder barrel with alayer of an aluminum containing metal having greater thickness than thebarrel wall, which comprises immersing the barrel in a bath of metal ofthe class consisting of aluminum and aluminum-base alloys at atemperature and for a time suiicient to form a film of ferro-aluminumalloy on said barrel, trans.. ferring the barrel without substantialcooling to a second bath of a molten metal of the class consisting ofaluminum and aluminum-base alloys maintained at a lower temperature thanthe irst bath to arrest further formation of the ferroaluminum alloy andclean the surface of Vsaid alloy lm, transferring the barrel withoutsubstantial cooling into a mold, and immediately casting a layer ofmetal of the class consisting of aluminum and aluminum-base alloys onthe wetted surface of said barrel to form the layer thereon.

5. The method of coating a ferrous metal engine cylinder barrel or thelike with a layer of an aluminum-containing metal of the classconsisting of aluminum and aluminum base alloys, said layer havinggreater thickness than-the barrel wall, which comprises immersing thebarrel in a bath of a molten aluminum containing metal at a temperatureand for a time suiiicient to form a ferro-aluminum alloy iilm on thebarrel, transferring the barrel without substantial cooling to a secondbath of an aluminum containing metal at a lower temperature than the rstbath to arrest further formation of the alloy, transferring the barrelwithout substantial cooling into a mold, immediately casting a layer ofan aluminum containing metal on the wetted surface of said barrel toform the layer thereon, and cooling the barrel interiorly while thealuminum containing metal layer is freezing to induce substantiallyuniform radial contraction thereof.

6. A method of coating a ferrous metal article with aluminum or aluminumalloys, which comprises forming an initial molten surface coating of ametal of the class consisting of aluminum and aluminum-base alloys onthe ferrous metal article by immersing said article in a bath of themolten metal and maintaining said article in contact with said bath fora period of time at the bath temperature only sufficient to form aferroaluminum film at the interface of said bath and said article andheat said article to above the melting point of the coating metal, andthen casting a molten metal of the class consisting of aluminum andaluminum-base alloys on said initial coating while said coating andarticle are at a sufliciently high temperature for said coating to bemolten and to be wetted by said casting metal to form a permanent bondtherewith.

7. A method of directly coating a ferrous metal article withaluminum-containing metal of the class consisting of aluminum andaluminum base alloys, which comprises heating the surface of saidarticle to be coated to a temperature not materially in excess of 1625F. and above the melting point of said aluminum-containing metal tocause the ferrous metal of said surface to alloy with the saidaluminum-containing metal in con- `said heated ferrous metal surface ofthe perature within the melting range of said initial coating, so thatsaid initial coating and said layer of molten metal will unite, andfreezing said coating and layer on said surface whereby said coating andlayer form a substantially homogeneous coating of saidaluminum-containing metal of the required thickness integrally bonded tosaid surface of the article through said ferro-aluminum alloy.

MARSHALL G. WHITFIELD. VICTOR SHESHUNOFF.

